One-Carbon Metabolism in Plants. Regulation of Tetrahydrofolate Synthesis during Germination and Seedling Development

Abstract

Tetrahydrofolate (THF) is a central cofactor for one-carbon transfer reactions in all living organisms. In this study, we analyzed the expression of dihydropterin pyrophosphokinase-dihydropteroate synthase (HPPK-DHPS) in pea (Pisum sativum) organs during development, and so the capacity to synthesize dihydropteroate, an intermediate in the de novo THF biosynthetic pathway. During seedling development, all of the examined organs/tissues contain THF coenzymes, collectively termed folate, and express the HPPK-DHPS enzyme. This suggests that each organ/tissue is autonomous for the synthesis of THF. During germination, folate accumulates in cotyledons and embryos, but high amounts of HPPK-DHPS are only observed in embryos. During organ differentiation, folate is synthesized preferentially in highly dividing tissues and in photosynthetic leaves. This is associated with high levels of the HPPK-DHPS mRNA and protein, and a pool of folate 3-to 5-fold higher than in the rest of the plant. In germinating embryos and in meristematic tissues, the high capacity to synthesize and accumulate folate correlates with the general resumption of cell metabolism and the high requirement for nucleotide synthesis, major cellular processes involving folate coenzymes. The particular status of folate synthesis in leaves is related to light. Thus, when illuminated, etiolated leaves gradually accumulate the HPPK-DHPS enzyme and folate. This suggests that folate synthesis plays an important role in the transition from heterotrophic to photoautotrophic growth. Analysis of the intracellular distribution of folate in green and etiolated leaves indicates that the coenzymes accumulate mainly in the cytosol, where they can supply the high demand for methyl groups.The synthesis of numerous biological compounds and the regulation of many metabolic processes require the addition or removal of one-carbon units (C1 metabolism). Tetrahydrofolate (THF) coenzymes mediate these C1 transfer reactions that are involved in several major cellular processes, including the synthesis of purines and thymidylate, amino acid metabolism, pantothenate synthesis, mitochondrial and chloroplastic protein biogenesis, and Met synthesis (Fig. 1). Met is the direct precursor of S-adenosyl-Met (Ado-Met), which in turn is the source of methyl units for the synthesis of a myriad of molecules such as choline, chlorophyll, or lignin (for reviews, see Cossins, 2000;Scott et al., 2000;Hanson and Roje, 2001). In plants, THF is also involved in the photorespiratory cycle, a specific pathway that occurs at very high rates in green leaves from C3 plants. Photorespiration relies on two THF-dependent enzymes present in the matrix space of leaf mitochondria, the Gly decarboxylase complex (GDC) and Ser hydroxymethyltransferase (SHMT; for reviews, see Oliver, 1994;Douce et al., 2001).THF is composed of three distinct parts, namely a pterin ring, a p-aminobenzoic acid, and a poly-Glu chain of variable length (1-8 residues). Its function is to bind, transport, and donate C1 units that differ i...

The Folylpolyglutamate Synthetase Plastidial Isoform Is Required for Postembryonic Root Development in Arabidopsis

“…In 7-d-old pea (Pisum sativum) seedlings, it was shown that the root meristem contained 5-fold more folates than the mature root. The elevated level of folates in the root meristem was strongly correlated with high mRNA and protein expression of dihydropterin pyrophosphokinase-dihydropteroate synthase, an enzyme that catalyzes reactions specific to THF synthesis (Jabrin et al, 2003). In maize (Zea mays) roots, expression of the gene encoding the bifunctional dihydrofolate reductase and thymidylate synthase, which is involved in de novo generation of folate and thymidylate (Cossins, 2000; Fig.…”

The Effect of α-Amanitin on the Arabidopsis Seed Proteome Highlights the Distinct Roles of Stored and Neosynthesized mRNAs during Germination

“…While the processes and genes regulating embryogenesis and maturation are being identified (Errampalli et al, 1991;Girke et al, 2000;BrocardGifford et al, 2003;Gallardo et al, 2003;Tzafrir et al, 2003), only very recent studies addressed the question of the exact requirements for germination, particularly in terms of de novo RNA and protein syntheses (Bradford et al, 2000;Bove et al, 2001;Gallardo et al, 2001Gallardo et al, , 2002aKoornneef et al, 2002;Potokina et al, 2002;van der Geest, 2002;Duque and Chua, 2003;Jabrin et al, 2003;Ogawa et al, 2003). Although it is widely documented that, in addition to the storage proteins, mature dry seeds accumulate during maturation a number of proteins that are involved in various cellular processes such as DNA replication and transcription, translation, metabolism, or defense mechanisms (e.g.…”

Folate Polyglutamylation is Required for Rice Seed Development

“…Interestingly, this study has shown that germination, the developmental stage reported to generate the greatest demand for folates (Jabrin et al 2003, Gambonnet et al 2001, was not affected in the fpgs03g mutant. This suggests that the remaining rice FPGS gene Os10g35940 can compensate for the lack of Os03g02030 activity during germination.…”

“…The reduction in monoglutamated folates may be a result of folate breakdown, as polyglutamylated protein-bound folates are less susceptible to oxidative breakdown (Suh et al 2001). All ten steps involved in the plant folate biosynthesis pathway have been described and characterised, and the expression of several enzymes involved in this pathway has been correlated with the folate pool and the carbon-one unit demands in various physiological situations (Basset et al, 2002(Basset et al, , 2004a(Basset et al, , 2004bJabrin et al, 2003). Indeed, this study has confirmed these previous observations, in that the reduction in folate abundance in tissues of the fpgs03g knockout line was shown to be correlated with an increased expression of a number of folate biosynthesis genes, namely GTPCHI, ADC synthase, ADC lyase and HPPK/DHPS compared to wild type (Fig.…”